This invention relates to a process for the separation of components of gaseous mixtures, and particularly relates to an improved selective adsorption process for the separation of high purity light gases, especially hydrogen or helium.
Cyclic swing adsorption or pressure swing adsorption hereinafter referred to as selective adsorption has been widely used for the recovery of lighter, less sorbed gases from a mixture with one or more heavier, more readily adsorbed gases.
The light gas desired in relatively more purified form is frequently hydrogen. Such hydrogen may be recovered from various hydrogen-containing gas mixtures such as purge streams from various synthesis processes involving hydrogen as a reactant, such as hydrogenations, synthesis of hydrocarbons, or as a product or by-product, such as dehydrogenation reactions as well as streams produced by the controlled combustion or reforming of hydrocarbons or from cracking of hydrocarbon feedstock. Another light gas which also is recovered in more purified form by the use of selective adsorption is helium which can be recovered from helium-containing natural gas compositions.
Selective adsorption systems generally involve passage of the feed gas mixtures through equipment comprising two or more adsorbers containing beds of molecular sieves or other adsorbents for the heavier components of the gas mixture. The selective adsorbers are arranged to operate in sequence with suitable lines, valves, timers and the like so there are established an adsorption period during which the heavier components of the feed gas mixture are adsorbed on the molecular sieve or other adsorbent, and a regeneration period during which the heavier components are desorbed and purged from the adsorbent to regenerate it for reuse. The widely used cyclic pressure processes provide a repressuring period during which the adsorber is brought substantially to adsorption period pressure. There are frequently added one or more guard adsorbers upstream from the primary selective adsorbers containing an adsorbent or adsorbents, which may differ from the primary adsorbent, used to adsorb any unusually heavy contaminants in a feed gas mixture which might tend to inactivate the primary adsorbent or render it difficult to regenerate, including such as vapors of normally liquid hydrocarbon and the like. The regeneration or purging of the primary selective adsorbers and the guard adsorbers in earlier selective adsorption processes was generally carried out by recycling at a lower pressure a part of the product stream of purified light gas.
Such processes of selective adsorption have been widely described in the literature and are familiar to those skilled in the art. Although these selective adsorption processes are effective to produce a relatively highly purified light gas product, they suffer from disadvantages. The greatest disadvantage is the relatively low percentage recoveries which are generally encountered when a substantial part of the light gas product is used for the regeneration of the primary adsorbers and/or guard adsorbers.
Recoveries of hydrogen gas generally range from about 50 to about 80%. A further disadvantage is the fact that the substantial portion of the light gas product used for purging and regeneration of the primary adsorbers and guard adsorbers cannot be economically recovered from the regenerating gas mixtures and such mixtures including the components separated by the guard adsorbers and primary adsorbers are generally disposed of by venting, flaring or employing as low grade fuels. Thus, any part of process reactants or products carried as vapors and adsorbed onto the guard adsorbers is lost to the principal process by the usual operation of the selective adsorption processes.
Subsequently improved selective adsorption processes have been suggested wherein the regeneration of the selective adsorbers and/or guard adsorbers employ a portion of the substantially purified void space light gas from a selective adsorber whose adsorptive capacity has not been fully occupied by adsorbed heavier components, but a portion of which adsorbent pores or "void space" is occupied by purified light gas, particularly that portion of the adsorbent at the downstream end of the adsorber. Such substantially purified light gas for regeneration can be stored in a separate vessel as taught by U.S. Pat. No. 3,142,547 of Marsh et al., or it can be used directly in a system of four or more sequenced selective adsorbers as taught by U.S. Pat. No. 3,430,418 of Wagner. Although such improved selective adsorption processes have reduced the amount of purified light gas product used for regeneration and repressuring and have increased the recoveries of the desired light gas product, the light gas disposed of along with the desorbed and purged components represents loss of desired product. In particular, any substantially purified light gas used to purge and regenerate the guard adsorbers by desorbing and purging the adsorbed heavier hydrocarbons is generally not recoverable and must be disposed of. Moreover when vented or flared these heavier hydrocarbons frequently represent vapors which are undesirable from the aspect of air pollution. Thus, further improved recoveries of purified light gas product and of the heavier, normally liquid aliphatic or aromatic hydrocarbons are desired.
It has also been suggested to employ a low temperature separator unit in conjunction with a pressure swing adsorption unit with recompression of the regenerating gas from the adsorbers and recycle to the low temperature separator unit as taught by U.S. Pat. No. 3,838,553 of Doherty. Such systems are complicated and costly, requiring an attendant refrigeration system for operation of the low temperature separation unit. Hence, such systems have not been widely used.
There has now been developed a selective adsorption process which produces a high purity hydrogen product and a high yield of such gas from feed gas mixture or mixtures containing hydrogen, and additionally provides for recovery of the guard adsorber purge gas and its content of hydrocarbon reactant or product. The process involves the use of a recovery system comprising a hydrogen selective permeator and a selective adsorption unit with recycle of a substantial portion of the regenerating hydrogen which is separated by the permeator unit to a guard adsorber of the selective adsorption unit and recovery thereof as added product hydrogen as well as recovery of hydrocarbon reactant and/or product adsorbed by the guard adsorber.